Glazing Inspection

1. A method of determining the optical quality of a glazing which includes at least one area having a reduced light transmission comprising: illuminating the glazing with a light source to form a shadowgraph image of the glazing on a virtual image plane, the virtual image plane being located between the light source and a camera; focusing the camera onto the virtual image plane; measuring the illumination of the glazing at a plurality of measurement points arranged in an array extending over the glazing; determining any deviation in illumination at those measurement points from a desired value at each measurement point; omitting from the array of measurement points the at least one area of reduced light transmission and ignoring the optical quality of the glazing in the area of reduced light transmission.

2. A method as claimed in claim 1 , wherein the illumination of the glazing with the light source comprises illuminating the glazing with a collimated light beam from a localised light source.

3. A method of forming a shadowgraph as claimed in claim 2 further comprising tilting the virtual image plane through use of a wedge prism positioned between the camera and the virtual image plane.

4. A method as claimed in claim 2 wherein the light source is an LED.

5. A method as claimed in claim 2 wherein the light is collimated by a lens optical system.

6. A method as claimed in claim 2 wherein the light is collimated by a mirror optical system.

7. A method as claimed in claim 1 , wherein the virtual image plane is positioned behind the glazing so that the glazing is positioned between the light source and the virtual image plane.

8. A method as claimed in claim 1 , wherein the focusing of the camera onto the virtual image plane comprises focusing onto the virtual image a CCD camera which records the shadowgraph image of the glazing that is stored in a computer associated with the CCD camera.

9. A method as claimed in claim 1 , wherein the glazing is illuminated under ambient light conditions.

10. A method according to claim 1 , further comprising: recording the shadowgraph image; processing the recorded shadowgraph image to determine an illumination value for each valid measurement point; constructing a reference image by scanning a convolution window point by point over the processed image and using a convolution filter to calculate a reference illumination value at points of the reference image which correspond to each point of the processed image by averaging the illumination values of the valid measurement points of the processed image covered by the convolution window; comparing the illumination value of each valid measurement point of the processed shadowgraph image with corresponding points of the reference image to determine the optical quality of the glazing.

11. A method as claimed in claim 10 wherein the convolution window is of constant area during the scanning operation.

12. A method as claimed in claim 10 including recording the reference image for comparison with the processed image.

13. A method as claimed in claim 10 wherein a valid measurement point is one in which the illumination value at that point is equal to or above a pre-set threshold.

14. A method as claimed in claim 10 wherein when the point of the convolution window for which the reference illumination is being calculated corresponds with a non-valid measurement point of the processed image, a reference illumination is not calculated.

15. A method as claimed in claim 10 wherein non-valid measurement points are not taken in account in the construction on the reference image.

16. A method as claimed in claim 10 wherein the same light source is used for the production of the shadowgraph image and in relation to calculating the reference image.

17. A method of determining the optical quality of a glazing which includes at least one area having a reduced light transmission comprising: illuminating the glazing with a light source to form a shadowgraph image of the glazing on a virtual image plane, the virtual image plane being located between the light source and a linescan camera, and the glazing moving relative to the linescan camera during the illuminating of the glazing to form the shadowgraph image; focusing the linescan camera onto the virtual image plane; measuring the illumination of the glazing at a plurality of measurement points arranged in an-array extending over the glazing; and determining any deviation in illumination at those measurement points from a desired value at each measurement point to determine the optical quality of the glazing; omitting from the array of measurement points the at least one area of reduced light transmission and ignoring the optical quality of the glazing in the area of reduced light transmission.

18. A method as claimed in claim 17 , wherein the light source is an LED.

19. A method as claimed in claim 17 , wherein the illuminating of the glazing with the light source comprises illuminating the glazing with the light source under ambient light conditions to form the shadowgraph image of the glazing on the virtual image plane.

20. A method of determining the optical quality of a glazing which includes at least one area having a reduced light transmission comprising: illuminating the glazing with a light source under ambient light conditions to form a shadowgraph image of the glazing on a virtual image plane, the virtual image plane being located between the light source and a camera; focusing the camera onto the virtual image plane; measuring the illumination of the glazing at a plurality of measurement points arranged in an array extending over the glazing; and determining any deviation in illumination at those measurement points from a desired value at each measurement point to determine the optical quality of the glazing; omitting from the array of measurement points the at least one area of reduced light transmission and ignoring the optical quality of the glazing in the area of reduced light transmission.